Relativity of one kilogram

In parts of the western Europe 16th century, a legal lever for measuring length and geodetic surveying of the land was made by law in front of the church. The instructions for calculating the length of one foot were: on Sunday, stand at the door of the church and when the service is over and the men start to leave the church, ask the 16 tall and small in order to stop. Ask them to put their left foot behind each other to get the length of 16 male left feet. The sixteenth part of that length shall be the true and lawful rate.

photo: wikipedia

As a measure, the foot was used in ancient times Kini, Ancient Greece, later in Rome, and survived through the Middle Ages to the present day. It was a kind of quasi-constant: men usually have a foot between 25 and 30 centimeters long, which is roughly the range between today's shoe numbers 39 and 47. But even with so much variation in the length of men's left feet, the foot was considered a more reliable measure, while others varied even more significantly from one part of Europe to another.

In the era of Charlemagne and the Carolingian Empire, which he ruled from 800 to 814, the most diverse measures were used in the west of Europe - mostly inherited from Rome - so diverse that they made trade and understanding between people difficult. Charlemagne decided to make efforts to establish some of them, such as the king's foot or the king's leg, throughout the empire. Although new measures were introduced after Charlemagne's death and later with the dismemberment of Europe, many of those advocated by Charlemagne persisted in France until the revolution of 1789.

Namely, during the French Revolution, the number of measurement units in use increased so much that it was very difficult to harmonize any measurement, and additionally, many of the measures had the same names, but not the same measurement. Demands to solve this question became very loud, and in constant discussions it was suggested that the unit must be based on a natural phenomenon, and not something that varies like the foot of a ruler or the feet of 16 pious men. The problem was solved when the Paris meridian was measured in 1799. This is how the meter was born. But what does it all have to do with it?

The Paris Meridian is an imaginary line that stretches from the North Pole to the South Pole, passing through Paris on the way. The line passes right over the Paris Observatory built in the 17th century at the time of Louis XIV and the founding of the French Academy of Sciences, when there was a strong development of science and education in Western Europe. The length of the meridian was measured for the first time in the second half of the 17th century, but these, as well as the results in the following measurements, varied, since - as it turns out - the Earth is not quite a perfect ball.

When, at the very end of the 18th century, two French astronomers - Jean-Baptiste Delambre and Pierre Mechen - measured the distance between the poles, their measurement was taken as the official constant for the meter calculation. The length of the imaginary line that stretches from the North Pole, passes through Paris and goes to the South Pole, and then returns to the North Pole on the other side of the planet, is 40.000.000 meters. One meter is like that, a quarter of the ten millionth part of this length. And no matter how many times you measure the meridian, it will be the same, which gave the meter some reliability.

In 1799, the National Assembly decided to introduce the meter as an official unit in France, but at the same time it introduced another crucially important unit - the kilogram. One kilogram is equal to the mass of one liter of water, that is, the amount of water that fits into a container shaped like a cube whose sides are one decimeter. The kilogram, along with the meter, will form the basis of the metric system.

Over the decades, the metric system slowly expanded - not only were new units introduced, but numerous countries gradually adopted it, so that in 1875, an international organization took over the role of guarding the pro-measure, i.e. standard. At this event, famously known as the Meter Agreement, or the Meter Convention, exactly 150 years ago, on May 20, 1875, the first 17 countries signed an agreement to accept the meter and the kilogram as official units.

The agreement on the meter meant that the kilogram and the meter were no longer "administered" by France, but that the job of preserving measurements passed into the hands of the newly established International Committee for Weights and Measures. The committee moved into the magnificent building of the Pavillon de Bretay near Paris in the suburb of Saint Cloud, which already at that time had an impressive history.

The magnificent castle and its nearby pavilion in the Paris suburb of Saint-Cloud, surrounded by a garden and overlooking the Seine, were built by Louis XIV in 1672. During the reign of Louis XVI, thanks to the efforts of the aristocrat Baron de Breteuil, the castle and the entire estate were transferred to Marie Antoinette in 1785. As a reward for his merits, she assigned the pavilion to the Baron for his use, naming it the Pavilion de Bretay. But the baron did not spend many happy years in the country. The French Revolution took place four years later, and just two and a half days before the fall of the Bastille, Louis XVI had just appointed Breteuil as chief minister. In the heated atmosphere, the Baron, like many others, fled outside of France, and the Pavilion de Bretay was confiscated and became state property.

After the revolution, the building got another famous owner - Napoleon, who thoroughly renovated it and changed its name to the Pavilion of Italy. He used it to host royal families visiting France, which became something of a tradition until the Franco-Prussian War. In 1870, the Castle of Saint Cloud was destroyed in the war, and the pavilion was badly damaged. Five years after this event, the pavilion was renovated on the occasion of the Meter Agreement and finally assigned to the International Committee for Weights and Measures, which is still at this address today. In the safe of the magnificent building, the meter and kilogram standards that were made at that time and which we know as the "prime-kilogram" and "prime-meter" are still kept. Although they no longer serve their original purpose, they are preserved as valuable historical artifacts.

New units were added to the metric system conceived in France, and their measurements became more and more precise. For the purposes of trade and navigation, in addition to length and mass, at that time a unit for time (s) was also necessary, and as knowledge increased and science developed, more basic units were introduced, namely for temperature (K), current (A), light intensity (cd) and amount of substance (mol). In 1921, the convention on the meter was supplemented by other areas of physics, so that in 1960 the entire system of units was called by its current name - the International System of Units (Systems international d'unites, SI).

However, as many calculations in the field of metrology have evolved, so the question of mass has become somewhat - relative. The one-kilogram pro-measure, which is a piece of metal weighing one kilogram, is kept in the safe of the Bretay Pavilion, under a glass container. In order to enter the room, it is necessary - like in movies about big robberies - that three people hold the key and turn them in the lock at the same time. But scientists are suspicious - could that piece of metal have gotten heavier or lighter over the course of 150 years? Did someone touch it and leave some dead skin cells on it, and what is their mass? Does the material deteriorate over time and in what way? In that case, a kilogram today is not the same as a kilogram 150 years ago - but for now we have no reliable method to find out.

And finally, does it even matter? If you buy fruit at the market, the mass of a couple of atoms here or there will not mean much to you. But in some areas, every atom counts - for example in the pharmaceutical industry, where medicine and poison sometimes differ only in dosage. A century later, the prototype kilogram and its copies no longer had identical mass, so scientists began to search for an alternative.

The goal was for the units to rely on constants in nature, and not on human artifacts, which - observed 150 years after the Meter Agreement - are not only unreliable, but can also be destroyed or lost, which happened with several standards, one of which is Serbian.

First, the second, meter and candela (light intensity) were redefined using fundamental physical constants, while the last revision of the system of international measures was in 2019, when the kilogram, ampere, kelvin and mole were also redefined by setting the correct numerical values ​​related to Planck's constant, elementary electric charge, Boltzmann's and Avogadro's constant. Thus, today we do not determine the meter by calculating the length of the meridian, but derive it from the speed of light, which is much more precise.

Meter in Serbia

The Principality of Serbia joined the Meter Agreement relatively quickly, four years after the first convention. The Assembly of the Principality of Serbia passed the first Law on Measures on the initiative of Milan Obrenović in 1873. According to the data of the Directorate for Measures and Precious Metals, this law introduced the decimal metric system of measures based on the French archival primary measures of the meter and the kilogram.

Also under the Ministry of Finance was created the State Service of Measures, the forerunner of today's Directorate, which was given the task of taking care of the control of measures. Along with the chief controller, this service had its own controllers in the districts and sections of Serbia who were in charge of inspecting and stamping the benchmarks. However, the authorities understood that order could not be brought to this area without acceding to the Agreement on Meters and without the first-order measures received by all signatories to the Meter Convention. That is why Serbia joined the Meta Convention in 1879.

The meter and kilogram prototypes arrived in Serbia two years later. The meter (number 30) is still kept in Belgrade, in the Directorate for Measures and Precious Metals. However, it too is now just a historical artifact. The prototype of the kilogram (number 11) that arrived in Belgrade in 1891 was used until 1925, when it was replaced by a new one (number 25), but was lost during the Second World War. The last one, number 33, made of stainless steel, has been in Serbia since 1956.